Heavy fighting in Congo's North Kivu province sent thousands of civilians fleeing yesterday as the rebel forces seized an army base and the headquarters of Virunga National Park, home to 200 or more of the world's 700 remaining mountain gorillas.

An unknown number of soldiers, rebels and civilians were killed, according to local people, who said the onslaught began at about 2am. Government troops raced north from Goma to reinforce a counterattack in the morning. One tank careered into fleeing civilians, killing three teenage boys, local reports said.

UN soldiers from India who tried to investigate the incident involving the boys were thwarted by people hurling stones.

As the fathers buried their sons outside Kibumba, shelling by army tanks could be heard about 12 miles from the base in Rumangabo.

Meanwhile, more than 50 forest rangers fled for Kibumba, according to Emmanuel de Merode, director of the Virunga park for the Congolese Wildlife Authority.

"The conflict on the ground is chaotic and dangerous, and we cannot allow our rangers to become targets," he said. Ten of the critically endangered gorillas were killed in the region last year, including two silverbacks, females and an infant, causing an international outcry.

The rebels who attacked the park's headquarters for the first time yesterday are loyal to the Congolese rebel leader, Laurent Nkunda. They claim that the Congolese army had attacked them and that they are only protecting the Tutsi minority. The rebels have occupied parts of Virunga National Park for nearly a year.

De Merode called the seizure of the headquarters "unprecedented, even in all the years of conflict in the region".

Yesterday's attack at Rumangabo marked the second time rebels had seized the army base since the summer, when Nkunda went on the offensive claiming that government troops had broken a January ceasefire agreement.

"There's heavy fighting. A lot of people have been killed - rebels, soldiers, civilians. We're lucky we got away," Jean-Baptiste Bushu Mbusho, a worker for an Italian aid agency, said yesterday.

More than 200,000 people have fled their homes since August, joining at least 1.2 million displaced when the conflict began in 2007, the UN said.

The UN force has failed to halt the fighting in the vast region of hills and forests in eastern Congo, and both elements in the conflict accuse the organisation of siding with the opposing side.

On Wednesday, near Kibumba, a hurled rock smashed the nose of a deputy commander in the Indian force. "He has had two major surgeries," said Sylvie van den Wildenberg, a UN spokeswoman.

A rapid reaction force was deployed yesterday with the UN appealing to both sides to cease fire. "But nobody is listening to us and they keep fighting," Van den Wildenberg said.

Many of the civilians who have been displaced are malnourished and some are dying of hunger, the UN World Food Program said last week. The Geneva-based agency is seeking $46m (£29m) in donations for food aid that will be needed to sustain refugees for a time. The fighting has jeopardised aid deliveries, and the UN agency said that some contractors were refusing to enter certain areas.